1. Field of the Invention
This application claims the benefit of Japanese Patent Application No. 2005-351750, filed Dec. 6, 2005, which is incorporated herein by reference.
The present invention relates to vapor compression refrigerating systems, and specifically, to vapor compression refrigerating systems using carbon dioxide refrigerant suitable for use in an air conditioning system for vehicles.
2. Description of Related Art
In a case in which carbon dioxide refrigerant, which is a natural-system refrigerant, is used as refrigerant for a vapor compression refrigerating system, a structure is disclosed in Japanese Patent Application No. H07-294033 A, wherein a pressure of refrigerant in a higher pressure-side line is adjusted by controlling a valve opening degree of an expansion device by an external control signal in order to improve an efficiency of a refrigerating cycle. In such a refrigerating system, a higher pressure-side pressure, realizes an optimum coefficient of performance of the refrigerating system, is calculated by referring to a temperature of refrigerant in the higher pressure side and the like, and the valve opening degree of the expansion device is controlled, so that the higher pressure-side pressure is optimized.
Further, as another method for improving an efficiency of a refrigerating system, a structure is disclosed, for example, in Japanese Patent Application No. H11-193967 A, wherein an internal heat exchanger is provided for exchanging heat between refrigerant at an exit side of a radiator and refrigerant at a suction side of a compressor. In such a refrigerating system having an internal heat exchanger, it is possible to suppress increases in higher pressure-side pressure by reducing the specific enthalpy of the refrigerant at the exit to the radiator as compared with a refrigerating system without the internal heat exchanger, thereby improving the coefficient of performance of the refrigerating system.
Thus, in a refrigerating system having an internal heat exchanger, refrigerant at an exit side of a radiator exchanges heat with refrigerant at a suction side of a compressor. Further, when using carbon dioxide refrigerant, although the temperature refrigerant discharged from a compressor is reduced by a radiator, if a temperature of an external fluid (for example, air), which exchanges heat with the refrigerant in the radiator, reaches or exceeds a certain temperature (for example, critical temperature of carbon dioxide), the refrigerant at the exit to the radiator may not condense and may remain at a supercritical condition. If the refrigerant pressure is reduced, and if the refrigerant is evaporated by an evaporator, the refrigerating capacity may be reduced significantly. Therefore, by the heat exchange between the refrigerant at the exit side of the radiator and the refrigerant at the suction side of the compressor, the refrigerating capacity may be increased or maintained, the higher pressure-side pressure may be reduced as compared with that of a refrigerating system having no internal heat exchanger, and the coefficient of performance of the refrigerating cycle may be increased.
In the above-described refrigerating system including an internal heat exchanger, however, because the refrigerant at an exit side of a radiator exchanges heat with the refrigerant at a suction side of a compressor, when the load of the refrigerating system is elevated, the superheating degree of the refrigerant at the suction side of the compressor also is elevated. As a result, the discharge temperature of the compressor also may be elevated to an undesirable level. Thus, it becomes necessary to increase the thermal resistance of the compressor itself in order to protect the compressor. Further, if the superheating degree of refrigerant drawn into the compressor increases, the power required for compressing operation of the compressor may increase. Therefore, in the above-described refrigerant system, the need to increase the thermal resistance of the compressor and the increased power required for compressor operation may be undesirable.